Power converter and controller for a power converter and manners of operation thereof

ABSTRACT

A power supply comprises a power converter having a transformer, a low side switch configured to draw current from a supply voltage through a primary winding of the transformer and a high side switch configured to couple the primary winding of the transformer to a snubber capacitor. A controller is configured to control the power converter by generating drive signals that control the opening and closing of the high side switch and the low side switch. The controller is configured to selectively control the high side switch according to various modes of operation depending on operating conditions such as input voltage and load power consumption. The modes of operation can include, for example, a mode in which the high side switch is closed and then opened once during each of the series of switching cycles and a mode of operation in which the high side switch is closed and then opened two times during each of the series of switching cycles.

This application claims priority of U.S. Provisional Application No. 63/039,728, filed Jun. 16, 2020, the entire contents of each of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to the field of power converters. More particularly, the present invention relates to a power converter and a controller for a power converter.

An off-line power supply receives power from an alternating-current (AC) power source and provides a voltage-regulated, direct-current (DC) output that can be used to power a load. An exemplary off-line power supply includes a power factor correction (PFC) stage and a DC-to-DC power converter stage. The PFC stage receives the AC input signal, performs rectification and maintains current drawn from the AC source substantially in phase with the AC voltage so that the power supply appears as a resistive load to the AC source. The DC-to-DC converter stage receives the rectified output of the PFC stage and generates the voltage-regulated, DC output that can be used to power the load. A voltage rectifier, such as a full-bridge rectifier and a smoothing capacitor, can be used in place of the PFC stage, in which case, the DC-to-DC stage can receive a rectified voltage from the voltage rectifier as its input. In either case, the input to the DC-to-DC stage is typically at higher voltage and is more loosely regulated than the output of the DC-to-DC stage.

A flyback power converter (or, more simply, a flyback converter) can be employed in a DC-to-DC power converter. A flyback converter employs a transformer that transfers energy from the input of the flyback converter to its output and provides electrical isolation between the input and output of the flyback converter. An input voltage, such as the rectified output voltage of a PFC stage or rectifier, is applied across the transformer primary winding by closing a switch; as a result, a primary winding current flows and magnetic flux in the transformer increases, thereby storing energy in the transformer. When the switch is opened, the voltage is removed and the primary winding current falls while magnetic flux drops. As a result, a current is induced in a secondary winding of the transformer. This induced current charges an output capacitor to generate an output voltage for powering a load.

Power supplies can be subjected to a variety of input voltage and loading conditions. It is important for such power supplies to operate efficiently so as to minimize power usage. Therefore, what are needed are improved techniques for a power supply that accommodate different operating conditions and that achieve efficient operation. What are further needed are such techniques for a power supply that employs a flyback power converter.

SUMMARY OF THE INVENTION

In accordance with an embodiment, a power supply comprises a power converter having a transformer, a low side switch configured to draw current from a supply voltage through a primary winding of the transformer and a high side switch configured to couple the primary winding of the transformer to a snubber capacitor. A controller is configured to control the power converter by generating drive signals that control the opening and closing of the high side switch and the low side switch. The controller is configured to selectively control the high side switch according to various modes of operation depending on operating conditions such as input voltage and load power consumption. The modes of operation can include, for example, a mode in which the high side switch is closed and then opened once during each of the series of switching cycles and a mode of operation in which the high side switch is closed and then opened two times during each of the series of switching cycles.

These and other embodiments are disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described with respect to particular exemplary embodiments thereof and reference is accordingly made to the drawings in which:

FIG. 1 illustrates a block schematic diagram of a two-stage, off-line power supply in accordance with an embodiment of the present invention;

FIG. 2 illustrates a flyback converter suitable for use in a DC-to-DC converter in accordance with an embodiment of the present invention;

FIG. 3 illustrates a voltage waveform for a flyback converter in a high line, light load mode of operation (also referred to herein as “first” mode of operation) in accordance with an embodiment of the present invention;

FIG. 4 illustrates a voltage waveform for a flyback converter in an alternative high line, light load mode of operation (also referred to herein as “second” mode of operation) in accordance with an embodiment of the present invention;

FIG. 5 illustrates a voltage waveform for a flyback converter in another alternative high line, light load mode of operation (also referred to herein as “combined” first and second mode of operation) in accordance with an embodiment of the present invention;

FIG. 6 illustrates a voltage waveform for a flyback converter in a further alternative high line, light load mode of operation (also referred to herein as “third” mode of operation) in accordance with an embodiment of the present invention;

FIG. 7 illustrates a voltage waveform for a flyback converter in a high line, heavy load mode of operation (also referred to herein as “first” mode of operation) in accordance with an embodiment of the present invention;

FIG. 8 illustrates a voltage waveform for a flyback converter in an alternative high line, heavy load mode of operation (also referred to herein as “combined” second and third mode of operation) in accordance with an embodiment of the present invention;

FIG. 9 illustrates a voltage waveform for a flyback converter in a low line, light load mode of operation (also referred to herein as “third” mode of operation) in accordance with an embodiment of the present invention;

FIG. 10 illustrates a voltage waveform for a flyback converter in an alternative low line, light load mode of operation (also referred to herein as “combined” first and second mode of operation) in accordance with an embodiment of the present invention;

FIG. 11 illustrates a voltage waveform for a flyback converter in an alternative low line, light load mode of operation (also referred to herein as “combined” second and third mode of operation) in accordance with an embodiment of the present invention;

FIG. 12 illustrates a voltage waveform for a flyback converter in a low line, heavy load mode of operation (also referred to herein as “third” mode of operation) in accordance with an embodiment of the present invention;

FIG. 13 illustrates a graph of switching frequency vs. input power for a flyback converter in accordance with an embodiment of the present invention;

FIG. 14 illustrates a flyback converter and control circuitry in accordance with an embodiment of the present invention;

FIG. 15 illustrates a controller integrated circuit for a DC-to-DC converter in accordance with an embodiment of the present invention;

FIG. 16 illustrates a high side driver control circuitry for a flyback converter in accordance with an embodiment of the present invention;

FIG. 17 illustrates a low side driver control circuitry for a flyback converter in accordance with an embodiment of the present invention;

FIG. 18 illustrates a differential signal converter for use in control circuitry for a flyback converter in accordance with an embodiment of the present invention;

FIG. 19 illustrates an oscillator for use in control circuitry for a flyback converter in accordance with an embodiment of the present invention;

FIG. 20 illustrates a comparator for use in control circuitry for a flyback converter in accordance with an embodiment of the present invention; and

FIG. 21 illustrates a timer for use in control circuitry for a flyback converter in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

The present invention is directed toward a power converter and a controller for a power converter and manners of operation thereof. The converter can be, for example, a flyback power converter employed in an offline switching power supply. In accordance with an embodiment of the present invention, the power converter employs two synchronously-operated transistor switches on the transformer primary side. A first of the transistor switches couples the transformer primary winding to a ground node and is referred to herein as the “low side” switch. A second of the two transistor switches couples the transformer primary winding to an input supply voltage via a snubber capacitor and is referred to herein as the “high side” switch. Switching (i.e. opening and closing) of the low side switch is controlled in a feedback loop to generate a regulated DC output voltage at the transformer secondary side. Switching of the high side switch is controlled, e.g., synchronously with the low side switch, in an effort to maximize efficiency of the power converter. In this context, efficiency refers to transfer of power from the input of the power converter to its output.

In accordance with an embodiment of the present invention, a controller for a power converter is provided. The controller is configured to control a power converter having a transformer, a low side switch and a high side switch. The low side switch draws current from an input supply voltage through a primary winding of the transformer. When the low side switch is opened, energy from the primary winding of the transformer is transferred to the snubber capacitor though the high side switch. The high side switch can also be used to discharge the snubber capacitor. The controller controls the opening and closing of the low side switch and the high side switch to generate a regulated output voltage.

In accordance with embodiments of the present invention, the low side switch is generally operated in accordance with: (1) a frequency control mode in which frequency of switching is controlled in a frequency control mode feedback loop to regulate the output voltage; and (2) a current control mode in which peak current in the primary winding of the transformer for each switch cycle is controlled in a current control mode feedback loop for regulating the output voltage.

In accordance with embodiments of the present invention, the high side switch is operated in a manner that is generally synchronous with the low side switch though this synchronous operation can be, at any instant, in accordance with one of several modes of synchronous operation. Under some conditions, the high side switch may be temporarily disabled (i.e. held open or “off”). A voltage (

) is generated at a node between the low side switch and the high side switch. During each switching cycle, the level of

rises after the low side switch is opened and, then, the level of

eventually falls from its peak level.

In accordance with an embodiment of a first mode of operation of the high side switch, the high side switch is closed when the voltage

is rising and is equal to, or very nearly equal to, a voltage level on the snubber capacitor. This voltage level on the snubber capacitor is generally the same as the level of the input voltage V_(IN) plus the level of a voltage V_(CSN) across the snubber capacitor. This closing of the high side switch discharges the snubber capacitor. The high side switch can be opened when the level of

falls to the level of V_(IN). This indicates that the snubber capacitor has been discharged.

In accordance with an embodiment of a second mode of operation of the high side switch, the high side switch is closed after the voltage

has fallen from its peak and is equal to, or very nearly equal to, a voltage level on the snubber capacitor, the voltage level on the snubber capacitor being generally the same as the level of the input voltage V_(IN) plus the level of a voltage V_(CSN) across the snubber capacitor. This closing of the high side switch discharges the snubber capacitor. The high side switch can be opened when the level of

falls to the level of V_(IN). This indicates that the snubber capacitor has been discharged. The first and second modes of operation can both be performed with the high side switch being operated under zero volt switching conditions. The high side switch is closed earlier in each switching cycle in the first mode of operation in comparison to the second mode of operation. This is because in the first mode of operation, the level of

is rising when the high side switch is closed, and in the second mode of operation, the level of

has fallen from its peak when the high side switch is closed.

In accordance with an alternative of the second mode of operation of the high side switch, the level of

is first allowed to oscillate one or more cycles of oscillation before the high side switch is closed. As in the second mode of operation described above, the high side switch is closed when the voltage

is equal to, or very nearly equal to, a voltage level on the snubber capacitor, the voltage level on the snubber capacitor being generally the same as the level of the input voltage V_(IN) plus the level of a voltage V_(CSN) across the snubber capacitor. The high side switch can be opened when the level of

falls to the level of V_(IN) which indicates that the snubber capacitor has been discharged.

The switching power supply can be, for example, operated with the high side switch being operated: in accordance with the first mode operation; in accordance with the second mode of operation; or in accordance with both of the first and second modes of operation together. When the first mode of operation and the second mode of operation are performed together, the high side switch is closed and opened and then closed and opened while the low side switch stays opened. In other words, the high side switch is cycled on and then off twice for each cycle of the low side switch.

In accordance with an embodiment of a third mode of operation of the high side switch, the high side switch is closed and then opened where at least the opening of the high side switch is without reference to, or in response to monitoring of, the level of

. In this mode of operation, the high side switch may be closed when the voltage

is is equal to, or very nearly equal to, a voltage level on the snubber capacitor. However, the high side switch may then be opened in response to a level transition of a fixed or adjustable duration pulse and without regard to the level of

. This pulse is generated during the portion of each switching cycle during which the low side switch is closed. The precise timing of generation and duration of the pulse signal can be controlled by the controller of the switching power supply. This third mode of operation can be entered, for example, when the input voltage to the flyback converter falls below a predetermined input voltage threshold or under light load conditions. In still another mode of operation of the high side switch, the high side switch is held open or off, while the body diode of the high side switching transistor performs passive rectification. This mode of operation can also be entered, for example, when the input voltage to the flyback converter falls below a predetermined input voltage threshold or under light load conditions.

The switching power supply can be, for example, operated with the high side switch being operated: in accordance with the first mode operation; in accordance with the second mode of operation; in accordance with the third mode operation; or in accordance with multiple modes of operation together. When two modes of operation are performed together (e.g., first and second modes or second and third modes), the high side switch is closed and opened and then closed and opened while the low side switch stays opened. In other words, the high side switch is cycled on and then off twice for each cycle of the low side switch.

The term “continuous conduction mode” or “CCM” means that current in the primary winding of the transformer of a power converter flows continuously throughout successive switching cycles. The term “discontinuous conduction mode” or “DCM” means that the current in the primary winding of the transformer is allowed to fall to zero for a period of time (i.e. “dead” time) during each switching cycle. The term “critical conduction mode” or “CRM” refers to operation at or near the boundary between CCM and DCM operating modes. In other words, during CRM, the current in the primary winding to allowed to fall to zero just before the current rises again.

Operation in CRM is useful during high loading conditions in order to achieve high efficiency. When loading is reduced, however, the level of current in the primary winding that is required to maintain a regulated output voltage is also reduced. As a result, operation at lower loading levels tends to revert to DCM. The level of the input voltage can also affect operation. For example, when the input voltage is higher, the “dead” time duration in DCM tends to increase, whereas, when the input voltage is lower, the “dead” time duration in DCM tends to decrease. Input voltage levels can vary based on the level of the AC line voltage. For example, different countries have adopted different AC line voltage levels as their respective standards. Loading can also affect the input voltage level; for example, increased current draw from an AC source can reduce its voltage level.

In an embodiment, under light loading conditions, the switching power supply is operated in accordance with DCM. Under light loading conditions, the switching power supply can be operated in the frequency control mode feedback loop to regulate the output voltage. Additionally, under light loading conditions, the high side switch can, for example, be operated in accordance with the first mode of operation of the high side switch or in accordance with the second mode of operation of the high side switch or in accordance with both the first and second modes of operation of the high side switch (the first and second modes being operative together) or in accordance with the second and third modes of operation of the high side switch (the second and third modes being operative together).

In an embodiment, under heavy loading conditions, the switching power supply is operated in accordance with CRM. Under heavy loading conditions, the switching power supply can be operated in the current control mode feedback loop to regulate the output voltage. Additionally, under heavy loading conditions, the high side switch can, for example, be operated in accordance with the second mode of operation of the high side switch or in accordance with the third mode of operation or in accordance with both the first and second modes of operation of the high side switch (the first and second modes being operative together).

In an embodiment, under transitional loading conditions, where the loading level is between the light and heavy loading conditions, the switching power supply preferably transitions its operation between the frequency control mode feedback loop and the current control mode feedback loop with hysteresis. Additionally, under transitional loading conditions, the switching power supply can be operated in DCM or CRM. This may also be in accordance with hysteresis such that the operation under transitional loading may depend upon whether the power supply operated in DCM or CRM just prior to entry into operation under transitional loading. Similarly, under transitional loading conditions, the high side switch is preferably operated in accordance with the mode (or modes) of operation of the high side switch in which it is operated just prior to entry into operation under transitional loading.

FIG. 1 illustrates a block schematic diagram of a two-stage, off-line power supply 100 in accordance with an embodiment of the present invention. As shown in FIG. 1 , a first stage 102 has an input coupled to alternating-current (AC) source. The first stage 102 performs rectification on the AC input signal. The first stage 102 can also perform power factor correction (PFC), in which case, the first stage 102 also maintains current drawn from the AC source substantially in phase with the AC voltage so that the power supply 100 appears as a resistive load to the AC source. The PFC stage can be replaced with a voltage rectifier, such as a full-bridge rectifier and a smoothing capacitor.

The first stage 102 generates a loosely regulated voltage, V_(DC), which is provided as input to a DC-to-DC converter 104. Using the input supply voltage V_(DC), the DC-to-DC converter stage 104 generates a voltage-regulated, DC output, V_(O), which can be used to power a load. The level of V_(DC) is preferably at a higher voltage and is more loosely regulated than the output, V_(O), of the DC-to-DC converter stage 104. The nominal level of the output, V_(DC), of the first stage 102 may be, for example, approximately 380 volts DC, while the voltage-regulated output, V_(O), of the DC-to-DC converter stage 104 may be, for example, approximately 15.0 volts DC. The input voltage, V_(AC), can be of different levels and frequencies, depending upon availability. For example, in the United States, 120 volts at 60 Hz is generally available, whereas, in China, 220 volts at 50 Hz is generally available.

FIG. 2 illustrates a flyback converter 150 in accordance with an embodiment of the present invention. The flyback converter 150 is suitable for use in a DC-to-DC converter of a switching power supply, such as the DC-to-DC converter 104 of FIG. 1 . The flyback converter 150 receives an input voltage from source V_(IN) that can be a rectifier output or PFC stage output, V_(DC), or that can be received from some other source, such as an electromagnetic interference (EMI) filter.

As shown in FIG. 2 , the input voltage source V_(IN) is coupled to a first terminal of a capacitor C_(SN) and to a first terminal of a primary winding of a transformer T₁. The capacitor C_(SN) functions as a snubber capacitor. A voltage V_(CSN) having polarity as shown in FIG. 2 is formed across the capacitor C_(SN). A second terminal of the primary winding of the transformer T₁ is coupled to a first terminal of a switch SW₁ (“low side” switch) and to a first terminal of a switch SW₂ (“high side” switch). A voltage

is formed at a node between the low side switch SW₁ and the high side switch SW₂ and at the second terminal of the primary winding of the transformer T₁. A second terminal of the switch SW₁ is coupled to a first ground node. A second terminal of the switch SW₂ is coupled to a second terminal of the capacitor C_(SN). The switch SW₁ is controlled by a signal LOWOUT while the switch SW₂ is controlled by a signal HIGHOUT.

The low side switch SW₁ and the high side switch SW₂ are each preferably implemented by a corresponding power MOSFET. Thus, a body diode is shown associated with each of switches SW₁ and SW₂.

A first terminal of a secondary winding of the transformer T₁ is coupled to an anode of a Zener diode D₁. A cathode of the diode D₁ is coupled to a first terminal of a capacitor C₁. A second terminal of the secondary winding of the transformer T1 is coupled to a second terminal of the capacitor C₁ and to a second ground node. The first and second ground nodes are preferably electrically isolated from each other.

The flyback converter 150 is operated by opening and closing the switches SW₁ and SW₂. The transformer T₁ transfers energy from the input of the flyback converter 150 to its output and provides isolation between the input and output of the flyback converter 150. In operation, when the switch SW₁ is closed (the switch is turned “ON”), voltage source V_(IN) is applied across the primary winding of the transformer T₁. As a result, a current in the primary winding and a magnetic flux in the transformer T₁ increases, which stores energy in the transformer T₁. When the switch SW₁ is then opened (the switch is turned “OFF”), the current in the primary winding and the magnetic flux drops. As a result, a current is induced in the secondary winding of the transformer T₁ that charges the capacitor C₁ with energy to generate an output voltage V_(O) for powering a load.

The level of power transferred to the load can be controlled by adjusting the switching duty cycle of the switch SW₁ (e.g., by controlling peak input current), the switching frequency of the switch SW₁, or both. Controlling the duty cycle is referred herein to as peak current control, whereas, controlling the switching frequency is referred to herein as frequency control.

When the switch SW₁ is opened and the switch SW₂ is in the closed position (the switch SW₂ is “ON”), the current in the primary winding of the transformer T₁ can pass through the switch SW₂ to the snubber capacitor C_(SN). Alternatively, when the switch SW₁ is opened and the switch SW₂ is in the open position (the switch SW₂ is “OFF”), the current in the primary winding of the transformer T₁ can pass through the body diode of the switch SW₂ to the snubber capacitor C_(SN). Closing the switch SW₂ can also discharge the snubber capacitor C_(SN) depending on its voltage level as compared to the level of

.

In a typical switching cycle, the high side switch SW₂ is preferably controlled such that it is open (OFF) when the low side switch SW₁ is closed (ON). Then, when the switch SW₁ is opened (OFF) and energy from the transformer T₁ transferred to the output capacitor C₁, the voltage

will become equal to or nearly equal to the voltage at the capacitor C_(SN). Under these conditions, the switch SW₂ can be briefly closed (ON). The switch SW₂ can, therefore, be operated under zero volt switching (ZVS) conditions. Closing the switch SW₂ equalizes the level of

to that of V_(IN)+V_(CSN). Then, once

and V_(IN)+V_(CSN) are substantially equalized, the switch SW₂ can be opened (OFF). The voltage

falls after the switch SW₂ is opened, such that when the switch SW₁ is closed, the voltage across it may be zero or nearly zero. Thus, the switch SW₁ can also be operated under zero volt switching (ZVS) conditions. The cycle then repeats. Operation of the switches SW₁ and SW₂ in accordance with ZVS, as described above, can be advantageous but is not always necessary.

To summarize, during a typical switching cycle, the low side switch SW₁ is turned off; then the high side switch SW₂ is turned on and then off before the low side switch SW₁ is turned on again. The cycle is then repeated (i.e. SW₁−off, SW₂−on, SW₂−off, SW₁−on, SW₁−off, . . . ). The high side switch SW₂ is thus turned on then off once (or twice as explained herein) before each low side switch SW₁ turn on event. Also, the high side switch SW₂ is turned on then off once (or twice) for each cycle of the low side switch SW₁ (while the low side switch SW₁ is off). In other words, each of the switches SW₁ and SW₂ is turned on and then off while the other switch is off.

In an embodiment, both switches SW₁ and SW₂ are operated under ZVS. In other embodiments, the switches SW₁ and SW₂ are not operated under ZVS or are operated partially under ZVS. For example, in accordance with an embodiment of the present invention, the high side switch SW₂ is opened at a time instant that depends upon the level of V_(IN). In this circumstance, the switch SW₂ may be opened under non-ZVS conditions because it is opened before or after the level of

is equal to V_(IN)+V_(CSN). In another example, the high side switch SW₂ can be opened in response to a fixed or adjustable duration pulse signal, as described in more detail herein; in this circumstance, the switch SW₂ may not be operated completely in accordance with ZVS.

The flyback converter 150 has a resonant switching frequency. The resonant frequency is dependent upon physical characteristics of the flyback converter 150, including the inductance value of the transformer T₁ primary winding and parasitic capacitance of the switches SW₁ and SW₂. When the switch SW₂ is closed, this introduces the capacitance of the snubber capacitor C_(SN) and therefore effectively changes the instantaneous resonant frequency of the flyback converter 150 while the switch SW₂ is closed.

The diode D₁ coupled to the transformer T₁ secondary winding operates as a freewheeling diode, allowing current in the secondary winding of the transformer T₁ to charge the capacitor C₁, and preventing discharge of the capacitor C₁ through the transformer T₁. The diode D₁ can alternatively be replaced with a switch that is operated synchronously with the switches SW₁ and SW₂ (synchronous rectification).

The switching, and particularly that of the high side switch SW₂, can be performed in accordance with various modes of operation, depending upon operating conditions. These operating conditions can include, for example, input voltage and loading. In an embodiment, the AC input voltage can be divided into two ranges: “high” and “low.” In this case, 220 volts AC input can be considered to be within the “high” range (the “high” range is also referred herein as “high line”) whereas 120 volts can be considered to be within the “low” range (the “low” range is also referred to herein as “low line”). The dividing line between the low and high input voltage ranges can be somewhere between 120 and 220 volts AC (e.g., around 170 volts AC). Similarly, the loading can be divided into two ranges: “light” and “heavy.” Loading refers to power level demanded by a load to which the power supply provides power. The power supply described herein can provide power to loads that have a wide variety of power requirements, including loads can require different power levels at different times.

Together, variations in the input voltage and load power operating conditions may result in at least four different possible “regions” of operation: (1) high line and light load; (2) high line and heavy load; (3) low line and light load; and (4) low line and heavy load. By changing operating modes in response to these operating conditions, efficiency of the power supply can be improved. While two ranges of input voltage and load power are described, it will be apparent that additional ranges can be provided. For example, rather than light and heavy loading, loading can be “light,” “moderate” and “heavy.” Similarly, one or more additional ranges can be provided for the input voltage. As explained herein, transitional “regions” of operation may also be provided for transitioning among the above-described regions of operation.

FIG. 3 illustrates a voltage waveform for a flyback converter in a high line, light load mode of operation (also referred to herein as “first” mode of operation) in accordance with an embodiment of the present invention. The waveform represents the level of

for two switching cycles. As shown in FIG. 3 , the switch SW₁ is closed (ON) at time t₀ so that the level of

is essentially zero volts. This causes current to flow in the primary winding of the transformer T₁ which charges the primary winding of the transformer T₁ with energy. The switch SW₁ is then opened (OFF) at time t₁. As a result, the level of

rapidly rises to a level above that of V_(IN). Current may then pass through the body diode of the switch SW₂ while energy from the transformer T₁ induces a current in the secondary winding of the transformer T₁ which charges the output capacitor C₁. Then, when the voltage

is rising and is equal to V_(IN)+V_(CSN), or nearly equal to V_(IN)+V_(CSN), the switch SW₂ is closed (ON) at time t₂. This tends to equalize the level of

towards that of V_(IN)+V_(CSN), discharging the capacitor C_(SN). Then, at time t₃ the switch SW₂ is opened (OFF). The switch SW₂ can be opened while the level of

is equal to or nearly equal to the level of V_(IN). The voltage

then falls before rising again and oscillating in accordance with the resonant frequency of the primary side of the converter. And, at time t₄ the switch SW₁ is closed (ON). This causes the level of

to fall to zero volts while a current again flows in the primary winding of the transformer T₁ and the above-described switching cycle repeats. As described above, the high side switch SW₂ is essentially operated under ZVS since the voltage across the switch SW₂ is zero or nearly so when it is closed and opened.

Also shown in FIG. 3 is a signal waveform for a signal “READYHIGHON.” In an embodiment of the present invention, the signal READYHIGHON must be a logic high voltage in order for the switch SW₂ to be closed. The period of time during which the signal READYHIGHON is logic high voltage is therefore a “window” of time during which the high side switch SW₂ can be closed. In other words, the signal READYHIGHON gives “permission” for the switch SW₂ to be closed. As explained herein, the signal READYHIGHON can be used to determine the mode of operation for the switch SW₂. As shown in FIG. 3 , the level of READYHIGHON is initially a logic low voltage which means that the switch SW₂ does not have permission to close. However, at a time between t₁ and t₂, the level of READYHIGHON transitions to a logic high voltage which gives the switch SW₂ permission to close. As explained above, the switch SW₂ does close at time t₂ when the voltage

is equal to V_(IN)+V_(CSN). Then, at the time t₃, the switch SW₂ is opened. Shortly after the time t₃ the signal READYHIGHON transitions to a logic high voltage.

FIG. 4 illustrates a voltage waveform for a flyback converter in an alternative high line, light load mode of operation (also referred to herein as “second” mode of operation) in accordance with an embodiment of the present invention. The waveform of FIG. 4 represents the level of

for two switching cycles. As shown in FIG. 4 , the switch SW₁ is initially closed (ON) at time t₀ so that the level of

is essentially zero volts. This causes current to flow in the primary winding of the transformer T₁ which charges the primary winding of the transformer T₁ with energy. The switch SW₁ is then opened (OFF) at time t₁. As a result, the level of

rapidly rises to a level above that of V_(IN). Current may then pass through the body diode of the switch SW₂ while energy from the transformer T₁ induces a current in the secondary winding of the transformer T₁ which charges the output capacitor C₁. The voltage

may then begin to oscillate in accordance with the resonant frequency of the converter. Then, when the voltage

is falling and is equal to V_(IN)+V_(CSN), or nearly equal to V_(IN)+V_(CSN), the high side switch SW₂ is briefly closed (ON) at time t₂. This equalizes the level of

towards that of V_(IN)+V_(CSN), discharging the capacitor C_(SN). Then at time t₃ when

is equal to or nearly equal to V_(IN) the switch SW₂ is opened (OFF). And, at time t₄ the switch SW₁ is again closed (ON). This causes the level of

to fall to zero volts while a current again flows in the primary winding of the transformer T₁ and the switching cycle repeats.

Because the voltage

is preferably equal to or nearly equal to V_(IN)+V_(CSN) when the switch SW₂ is closed, the switch SW₂ can be closed under zero volt switching (ZVS) conditions. And, because the voltage

is equal to or nearly equal to V_(IN) when the switch SW₂ is opened, the switch SW₂ can be opened under ZVS conditions. By controlling the high side switch SW₂ synchronously with ZVS, this provides for more efficient operation, for example, by avoiding losses caused by non-ZVS switching, and allows for operation at higher switching frequencies than otherwise which also tends to increase efficiency of the flyback converter.

Also shown in FIG. 4 is a signal waveform for the signal READYHIGHON. In contrast to FIG. 3 , the READYHIGHON signal of FIG. 4 does not give permission for the switch SW₂ to be closed until after the level of

has risen beyond the level of V_(IN)+V_(CSN) and has begun to oscillate. More particularly, in FIG. 4 , the level of READYHIGHON transitions to a logic high voltage just prior to the time t₂ and then returns to a logic low voltage shortly after the time t₃. Therefore, by delaying activation of the signal READYHIGHON as compared to FIG. 3 , this results in the switch SW₂ being closed later in the switching cycle in FIG. 4 (as in the “second mode”) as compared to FIG. 3 (which shows the “first mode”). However, in both FIGS. 3 and 4 , the precise timing of closing and opening the switch SW₂ within the “window” of time provided by the signal READYHIGHON can depend on monitoring the level of

in order to maintain ZVS.

The

waveform shown in FIG. 4 assumes that the closing of switch SW₂ is delayed such that the voltage

tends to oscillate. More particularly, FIG. 4 shows two oscillations occurring prior to the closing of switch SW₂ at time t₂. This is referred to herein as “valley switching.” It will be apparent that greater or fewer oscillations can occur while maintaining zero volt switching so long as the switch SW₂ is closed at a time when

is equal to or nearly equal to V_(IN)+V_(CSN). For example, the switch SW₂ may be closed as soon as the falling voltage

first becomes equal to V_(IN)+V_(CSN), or nearly equal to V_(IN)+V_(CSN). This is referred to herein as “quasi-resonant” or “first hill” switching. In either case, zero volt switching can be maintained if the switch SW₂ is closed at a time when

is equal to or nearly equal to V_(IN)+V_(CSN). The flyback converter 150 can selectively operate in accordance with quasi-resonant switching or valley switching, depending upon conditions such as switching frequency, loading conditions, component values, and so forth, in order to regulate the output voltage. These possibilities can be achieved by adjusting the timing of the level transitions of the signal READYHIGHON.

FIG. 5 illustrates a voltage waveform for a flyback converter operating in another alternative high line, light load mode of operation (also referred to herein as “combined” first and second mode of operation) in accordance with an embodiment of the present invention. The waveform of FIG. 5 represents the level of

for two switching cycles. As shown if FIG. 5 , the switch SW₁ is initially closed (ON) at time t₀ so that the level of

is essentially zero volts. This causes current to flow in the primary winding of the transformer T₁ which charges the primary winding of the transformer T₁ with energy. The switch SW₁ is then opened (OFF) at time t₁. As a result, the level of

rapidly rises to a level above that of V_(IN). Then, when the voltage

is rising and is equal to V_(IN)+V_(CSN), or nearly equal to V_(IN)+V_(CSN), the switch SW₂ is closed (ON) at time t₂. This equalizes the level of

towards that of V_(IN)+V_(CSN), discharging the capacitor C_(SN). Then, at time t₃ the switch SW₂ is opened (OFF). The switch SW₂ can be opened while the level of

is equal to the level of V_(IN). The voltage

then falls before rising again and oscillating in accordance with the resonant frequency of the primary side of the converter. Therefore, while the switch SW₁ is closed, the switch SW₂ is operated in accordance with the first mode of operation as described above in connection with FIG. 3 .

Referring to FIG. 5 , after the switch SW₂ is closed (at time t₂) and then opened (at time t₃), the level of

falls and then may begin to oscillate. When the level of

is falling and is equal to, or very nearly equal to, the voltage V_(IN)+V_(CSN), then the switch SW₂ can be closed for a second time during this same switching cycle. This is shown in FIG. 5 at time t₄. Then the switch SW₂ can be opened for a second time during this same switching cycle. This is shown in Figure at time t₅, when the level of

is equal that of V_(IN). This second cycling of the switch SW₂ can therefore be in accordance with the second mode of operation of the switch SW₂, as described above in connection with FIG. 4 . This second cycling of the high side switch SW₂ occurs while the low side switch SW₁ remains open. Therefore, the high side switch SW₂ is closed and opened twice during each switching cycle of the low side switch SW₁.

FIG. 5 also shows a signal waveform for the signal READYHIGHON. This waveform shows two windows of time during which the switch SW₂ has permission to be closed during each switching cycle. As in FIG. 3 , the first window commences when signal READYHIGHON transitions to a logic high voltage shortly before the time t₂. The first window ends when the signal READYHIGHON transitions to a logic low voltage shortly after the time t₃. This first window is comparable to the window of FIG. 3 . The second window commences when signal READYHIGHON transitions to a logic high voltage shortly before the time t₄. The first window ends when the signal READYHIGHON transitions to a logic low voltage shortly after the time t₅. This second window is comparable to the window of FIG. 4 . Thus, as shown by the two windows of time in FIG. 5 , the switch SW₂ is given permission to be closed and opened twice during each switching cycle of the low side switch SW₁. Additionally, the precise timing of each of these two cycles of the switch SW₂ within each window can be based on monitoring

and, therefore, ZVS can be maintained for both cycles.

The voltage waveform of FIG. 5 illustrates valley switching operation of the switch SW₂ when the switch SW₂ is closed for a second time during each switching cycle. FIG. 5 shows five oscillations occurring prior to the closing of switch SW₂. It will be apparent that greater or fewer oscillations can occur prior to the closing of switch SW₂. For example, in the case of two oscillations, the switch SW₂ can be closed as illustrated in FIG. 3 . In accordance with the embodiment of FIG. 5 , the high side switch SW₂ is closed and opened twice during the portion of each switching cycle in which the low side switch SW₁ remains open. The high side switch SW₂ can be closed a first time while the level of

is rising and can be closed the second time after the level of

has fallen from its peak. Alternatively, or additionally, high side switch SW₂ can be closed a first time before the level of

oscillates one or more cycles and can be closed the second time after the level of

oscillates. These possibilities can be achieved by adjusting the timing of the level transitions of the signal READYHIGHON.

As shown in FIG. 5 , at time t₅ the switch SW₁ is closed (ON). This causes the level of

to fall to zero volts while a current again flows in the primary winding of the transformer T₁. The switching cycle then repeats.

FIG. 6 illustrates a voltage waveform for a flyback converter in a further alternative high line, light load mode of operation (also referred to herein as “third” mode of operation) in accordance with an embodiment of the present invention. The third mode of operation is somewhat similar to the first mode of operation of the high side switch SW₂ (which is illustrated in FIG. 3 ) in that closing the high side switch SW₂ can be performed in a similar manner. However, in the third mode of operation, the high side switch SW₂ is opened without reference to, or needing to monitor, the level of

. In this mode of operation, the high side switch can be opened in response to a level transition of a fixed or adjustable duration pulse signal (e.g. the signal READYHIGHON) while the low side switch SW₁ remains closed.

More particularly, the signal READYHIGHON which is also shown in FIG. 6 can be used to control the precise timing of opening the high side switch SW₂. Unlike the windows of time formed by the signal READYHIGHON in FIGS. 3 and 4 , the window of time formed by the signal READYHIGHON closes before the level of

is equal that of V_(IN). This closing of the window, therefore, causes the switch SW₂ to open immediately when the signal READYHIGHON transitions to a logic low voltage. In other words, the READYHIGHON signal overrides the comparison of

to V_(IN) that takes place in FIGS. 3 and 4 . The duration of the period during which the high side switch SW₂ is closed in this third mode of operation shown in FIG. 6 is adjustable by controlling the duration of the READYHIGHON pulse. The duration of the period during which the high side switch SW₂ is closed in this third mode of operation is also generally shorter than that the duration in which the high side switch is closed in the first mode of operation (FIG. 3 ).

As shown in FIG. 6 , the signal READYHIGHON transitions to a logic high voltage shortly before the time t₂. Then, as soon as the level of

is equal to, or very nearly equal to, the voltage V_(IN)+V_(CSN), then the switch SW₂ can be closed. This occurs at the time t₂. The signal READYHIGHON transitions to a logic low voltage at the time t₃. This causes the high side switch SW₂ to be immediately opened at the time t₃. Therefore, the switch SW₂ remains closed during the time period between t₂ and t₃. The duration of the time period t₂-t₃ illustrated in FIG. 6 can be dependent upon the level of V_(IN). For example, FIG. 6 can represent the highest expected level of V_(IN), for example, 380 volts DC. However, this time period can be adjusted based on the level of V_(IN). For example, when V_(IN) is lower than 380 volts DC (e.g., 240 volts DC), then the level of READYHIGHON can transition to a logic low voltage sooner and the switch SW₂ can be opened sooner in the switching cycle thereby moving the time t₃ closer to t₂ and shortening the duration of the time period between t₂ and t₃. The amount by which the time period t₂-t₃ is shortened can be proportional to the change in the level of V_(IN).

As explained herein, the switch SW₂ is preferably closed under ZVS in accordance with the third mode of operation shown in FIG. 6 . However, the switch SW₂ can be opened under non-ZVS conditions. Opening the high side switch SW₂ under non-ZVS conditions helps to improve efficiency by not fully discharging the snubber capacitor C_(SN) (FIG. 2 ) under such conditions.

In an alternative of the third mode of operation of the high side switch, the high side switch SW₂ is held off, while the body diode of the high side switching transistor SW₂ still performs rectification. This can be accomplished by maintaining the level of READYHIGHON a logic low voltage throughout the switching cycle.

In a further alternative of the third mode of operation, the closing of the high side switch SW₂ can be in response to the signal READYHIGHON and independent of the level of

. This can be accomplished by transitioning the level of READYHIGHON before the changing level of

would otherwise cause the high side switch SW₂ to close.

FIG. 7 illustrates a voltage waveform for a flyback converter in a high line, heavy load mode of operation (also referred to herein as “first” mode of operation) in accordance with an embodiment of the present invention. The waveform represents the level of

for two switching cycles. As shown in FIG. 7 , the switch SW₁ is closed (ON) at time t₀ so that the level of

is essentially zero volts. This causes current to flow in the primary winding of the transformer T₁ which charges the primary winding of the transformer T₁ with energy. The switch SW₁ is then opened (OFF) at time t₁. As a result, the level of

rapidly rises to a level above that of V_(IN). Current may then pass through the body diode of the switch SW₂ while energy from the transformer T₁ induces a current in the secondary winding of the transformer T₁ which charges the output capacitor C₁. Then, when the voltage

has risen to become equal to V_(IN)+V_(CSN), or nearly equal to V_(IN)+V_(CSN), the switch SW₂ is closed (ON) at time t₂. This tends to equalize the level of

towards that of V_(IN)+V_(CSN), discharging the capacitor C_(SN). The level of

gradually falls. Then, at time t₃ the switch SW₂ is opened (OFF). The switch SW₂ can be opened while the level of

is equal to or nearly equal to the level of V_(IN). The voltage

ontinues to fall more rapidly. And, at time t₄ the switch SW₁ is closed (ON). This causes the level of

to fall to zero volts while a current again flows in the primary winding of the transformer T₁ and the above-described switching cycle repeats. Operation in this mode is comparable to that of FIG. 3 though the heavier loading may affect the resulting waveform of

.

FIG. 7 shows that the signal READYHIGHON transitions to a logic high voltage shortly before the time t₂ and then the signal READYHIGHON transitions to a logic low voltage shortly after the time t₃. Thus, the precise timing of closing and opening the high side switch SW₂ can be determined by monitoring

.

FIG. 8 illustrates a voltage waveform for a flyback converter in an alternative high line, heavy load mode of operation (also referred to herein as “combined” mode of operation) in accordance with an embodiment of the present invention. The waveform of FIG. 8 represents the level of

for two switching cycles. As shown if FIG. 8 , the switch SW₁ is initially closed (ON) at time t₀ so that the level of

is essentially zero volts. This causes current to flow in the primary winding of the transformer T₁ which charges the primary winding of the transformer T₁ with energy. The switch SW₁ is then opened (OFF) at time t₁. As a result, the level of

rapidly rises to a level above that of V_(IN). Then, when the voltage

is rising and is equal to V_(IN)+V_(CSN), or nearly equal to V_(IN)+V_(CSN), the switch SW₂ is closed (ON) at time t₂. This equalizes the level of

towards that of V_(IN)+V_(CSN), discharging the capacitor C_(SN). Then, at time t₃ the switch SW₂ is opened (OFF).

FIG. 8 also shows the signal READYHIGHON. Here, the signal READYHIGHON transitions to a logic high voltage shortly before the time t₂. This allows the precise timing of closing the high side switch SW₂ to be determined by monitoring

. Thus, SW₂ can be closed in accordance with ZVS. The signal READYHIGHON transitions to a logic low voltage at the time t₃ which causes the high side switch SW₂ to immediately open irrespective of the level of

. Accordingly, the high side switch SW₂ may be opened at the time t₃ under non-ZVS conditions.

The voltage

then falls before rising again and oscillating in accordance with the resonant frequency of the primary side of the converter. Referring to FIG. 8 , after the switch SW₂ is closed (at time t₂) and then opened (at time t₃), the level of

falls and may also begin to oscillate. Shortly before the time t₄, the level of READYHIGHON transitions to a logic high voltage. When the level of

is falling and is equal to, or very nearly equal to, the voltage V_(IN)+V_(CSN), then the switch SW₂ can be closed for a second time during this same switching cycle. This is shown in FIG. 8 at time t₄. Then the switch SW₂ can be opened for a second time during this same switching cycle. This is shown in Figure at time t₅, when the level of

is equal that of V_(IN). Shortly after the time t₅, the level of READYHIGHON transitions to a logic low voltage. This second cycling of the high side switch SW₂ occurs while the low side switch SW₁ remains open. Therefore, the high side switch SW₂ is opened and closed twice during each switching cycle of the low side switch SW₁. The first cycle of the high side switch SW₂ can be terminated by the signal READYHIGHON as in the third mode of operation (rather than by monitoring

). The second cycle of the high side SW₂ is terminated based on monitoring the level of

.

Thus, similarly to FIG. 5 , the READYHIGHON signal of FIG. 8 shows two windows of time during which the switch SW₂ has permission to be closed during each switching cycle. The first window commences when signal READYHIGHON transitions to a logic high voltage shortly before the time t₂. However, unlike FIG. 5 , the first window preferably ends when the signal READYHIGHON transitions to a logic low voltage at the time t₃. This first window is comparable to the window of FIG. 6 (third mode). The second window commences when signal READYHIGHON transitions to a logic high voltage shortly before the time t₄. The first window ends when the signal READYHIGHON transitions to a logic low voltage shortly after the time t₅. This second window is comparable to the window of FIG. 4 (second mode). Thus, as shown by the two windows of time in FIG. 8 , the switch SW₂ is given permission to be cycled (i.e. closed and opened) twice during each switching cycle of the low side switch SW₁.

FIG. 9 illustrates a voltage waveform for a flyback converter in a low line, light load mode of operation (also referred to herein as “third” mode of operation) in accordance with an embodiment of the present invention. This mode of operation of the high side switch SW₂ is comparable to that of FIG. 6 . In this mode of operation, the high side switch SW₂ is closed based on monitoring the level of

and is then opened without reference to, or monitoring of, the level of

. In this mode of operation, the high side switch is opened in response to a fixed or adjustable duration pulse signal (e.g., the signal READYHIGHON) while the low side switch SW₁ remains closed. As explained in connection with the exemplary waveform of FIG. 6 , the duration of the time period t₂-t₃ illustrated in FIG. 9 can be dependent upon the level of V_(IN). And, as shown in FIG. 9 , the signal READYHIGHON transitions to a logic high voltage just prior to the time t₂. The high side switch SW₂ opens at the time t₂. Then, at the time t₃, the signal READYHIGHON transitions to a logic low voltage which causes the switch SW₂ to immediately open. Thus, the switch SW₂ can opened under non-ZVS conditions.

In an alternative of this mode of operation of the high side switch, the high side switch SW₂ is held off while the body diode of the high side switching transistor SW₂ performs rectification. This can be accomplished by withholding the READYHIGHON signal for the duration of the switching cycle.

FIG. 10 illustrates a voltage waveform for a flyback converter in an alternative low line, light load mode of operation (also referred to herein as “combined” first and second mode of operation) in accordance with an embodiment of the present invention. This mode of operation of the high side switch SW₂ is comparable to that of FIG. 5 . The waveform of FIG. 10 represents the level of

for two switching cycles. As shown if FIG. 10 , the switch SW₁ is initially closed (ON) at time t₀ so that the level of

is essentially zero volts. This causes current to flow in the primary winding of the transformer T₁ which charges the primary winding of the transformer T₁ with energy. The switch SW₁ is then opened (OFF) at time t₁. As a result, the level of

rapidly rises to a level above that of V_(IN). Then, when the voltage

is rising and is equal to V_(IN)+V_(CSN), or nearly equal to V_(IN)+V_(CSN), the switch SW₂ is closed (ON) at time t₂. This equalizes the level of

towards that of V_(IN)+V_(CSN), discharging the capacitor C_(SN). Then, at time t₃ the switch SW₂ is opened (OFF). The switch SW₂ can be opened while the level of

is equal to the level of V_(IN). The voltage

then falls before rising again and oscillating in accordance with the resonant frequency of the primary side of the converter.

After the switch SW₂ is closed (at time t₂) and then opened (at time t₃), the level of

falls and may also begin to oscillate. When the level of

is falling and is equal to, or very nearly equal to, the voltage V_(IN)+V_(CSN), then the switch SW₂ can be closed for a second time during this same switching cycle. This is shown in FIG. 10 at time t₄. Then the switch SW₂ can be opened for a second time during this same switching cycle. This is shown in Figure at time t₅, when the level

is equal that of V_(IN). This second cycling of the high side switch SW₂ occurs while the low side switch SW₁ remains open. Therefore, the high side switch SW₂ is opened and closed twice during each switching cycle of the low side switch SW₁.

FIG. 10 also shows a signal waveform for the signal READYHIGHON. This waveform shows two windows of time during which the switch SW₂ has permission to be closed during each switching cycle. As in FIGS. 3 and 5 , the first window commences when signal READYHIGHON transitions to a logic high voltage shortly before the time t₂. The first window ends when the signal READYHIGHON transitions to a logic low voltage shortly after the time t₃. The second window commences when signal READYHIGHON transitions to a logic high voltage shortly before the time t₄. The first window ends when the signal READYHIGHON transitions to a logic low voltage shortly after the time t₅. Thus, as shown by the two windows of time provided by the signal READYHIGHON in FIG. 10 , the switch SW₂ is given permission to be closed and opened twice during each switching cycle of the low side switch SW₁. Additionally, the timing each of these two cycles of the switch SW₂ within each window can be based on monitoring

and, therefore, ZVS can be maintained for both cycles.

FIG. 10 shows five oscillations occurring prior to the closing of switch SW₂. It will be apparent that greater or fewer oscillations can occur prior to the closing of switch SW₂.

FIG. 11 illustrates a voltage waveform for a flyback converter in an alternative low line, light load mode of operation (also referred to herein as a “combined” second and third mode of operation) in accordance with an embodiment of the present invention. FIG. 11 differs from FIG. 10 in that the signal READYHIGHON in FIG. 11 transitions to a logic low voltage at the time t₃ which causes the high side switch SW₂ to immediately open. Therefore, the switch SW2 may be opened at the time t₃ in accordance with non-ZVS conditions. In FIG. 11 the signal READYHIGHON transitions to a logic high voltage prior to the time t₂ so that the switch SW₂ may be closed based on monitoring the level of

and in accordance with ZVS. Similarly to FIG. 10 , the signal READYHIGHON transitions to a logic high voltage prior to the time t4 and returns to a logic low voltage after the time t5. Therefore, the precise timing for opening and closing the switch SW₂ in the second cycle of the switch SW₂ can be based on monitoring the level of

and under ZVS conditions.

FIG. 12 illustrates a voltage waveform for a flyback converter in a low line, heavy load mode of operation (also referred to herein as “third” mode of operation) in accordance with an embodiment of the present invention. This mode of operation of the high side switch SW₂ is comparable to that of FIGS. 6 and 9 . In this mode of operation, the high side switch SW₂ is opened without reference to, or monitoring of, the level of

. In this mode of operation, the high side switch SW₂ can instead be opened in response to a level change in a fixed or adjustable duration pulse signal (e.g. the signal READYHIGHON) while the low side switch SW₁ remains closed. The duration of the period during which the high side switch is closed in this mode of operation is generally shorter than that the duration in which the high side switch is closed in other modes of operation (e.g. FIG. 3 ). As shown in FIG. 12 , the signal READYHIGHON transitions to a logic high voltage shortly before the time t₂. Then, as soon as the level of

is equal to, or very nearly equal to, the voltage V_(IN)+V_(CSN), then the switch SW₂ can be closed. This occurs at the time t₂. The signal READYHIGHON transitions to a logic low voltage at the time t₃. This causes the high side switch SW₂ to be immediately opened. Therefore, the switch SW₂ remains closed during the time period between t₂ and t₃. As explained in connection with the exemplary waveform of FIGS. 6 and 9 , the duration of the time period t₂-t₃ illustrated in FIG. 12 can be dependent upon the level of V_(IN).

In an alternative of this mode of operation of the high side switch, the high side switch SW₂ is held off, while the body diode of the high side switching transistor still performs rectification. This can be accomplished by withholding the READYHIGHON signal for the duration of the switching cycle.

FIG. 13 illustrates a graph of switching frequency vs. input power for a flyback converter in accordance with an embodiment of the present invention. Switching frequency f_(sw) is plotted on the vertical axis while input power, as measured by the feedback error signal V_(EAO) (e.g., FIG. 14 ) is plotted on the horizontal axis. As shown in FIG. 13 , the flyback converter can operate in different regions of operation dependent upon load power: (1) a very light loading region of operation 10; (2) a light loading region of operation 12; (3) a transitional loading region of operation 14; and (4) a heavy loading region of operation 16. While four regions 10, 12, 14, and 16 of operation are illustrated based on loading, it will be apparent that more or fewer regions can be implemented.

When the feedback error signal V_(EAO) is below a low threshold, for example, 0.75 volts, this indicates very light loading. In this region of operation 10, the flyback converter can be operated in “burst” or “kick” mode. In such a mode, switching (of switches SW₁ and SW₂) can be paused between “bursts” of switching for increased efficiency. Once the level of V_(EAO) rises above the low threshold, the flyback converter enters the light loading region of operation 12. In this region 12, the flyback converter can be operated in frequency control mode in which the switching frequency (of the low side switch SW₁) is modulated in a feedback loop to regulate the output voltage V_(O). Once the level of V_(EAO) surpasses a medium threshold of, for example, 2.0 volts, then the flyback converter enters the transitional loading region 14. In this region 14, operation in frequency control mode can continue until the level of V_(EAO) surpasses a high threshold of, for example, 2.5 volts.

Once the level of V_(EAO) surpasses the high threshold of, for example, 2.5 volts, then the flyback converter enters heavy loading region of operation 16. In this region 16, the switching frequency and is clamped to predetermined value f_(clamp) and the flyback converter enters a current control mode. In the current control mode, the peak current level (in the transformer T₁ primary winding) for each switching cycle, as sensed by a current sensing signal Isense (e.g. FIG. 14 ) is controlled in a feedback loop to regulate the output voltage V_(O). As the power level rises, the switching frequency can additionally be reduced, as shown in FIG. 13 by the downward sloping line in the regions 14 and 16. As the power level approaches full load, voltage regulation is still preferably performed primarily through the current control feedback loop.

As shown in FIG. 13 , the switch frequency vs. power curve is discontinuous. When transitioning from frequency control to current control (e.g., when V_(EAO) rises above 2.5 volts), the switching frequency is suddenly increased and the peak current in the transformer primary winding is, at the same time, reduced. Conversely, when transitioning from current control to frequency control (e.g., when V_(EAO) falls below 2.0 volts), the switching frequency is suddenly reduced and the peak current in the transformer primary winding is, at the same time, increased. In both modes, negative feedback is employed to regulate the output voltage V_(O). It is therefore important that operation of the flyback converter remains stable as it transitions between the frequency and current control modes of operation.

The switch frequency vs. power curve of FIG. 13 shows hysteresis in the transitions between frequency control and current control. That is, once the level of V_(EAO) rises above 2.5 volts and the switching converter transitions from the frequency control mode to the current control mode, the level of V_(EAO) must then fall below 2.0 volts in order to return to the frequency control mode. Similarly, once the level of V_(EAO) falls below 2.0 volts and the switching converter transitions to the frequency control mode from the current control mode, the level of V_(EAO) must then rise above 2.5 volts in order to return to the current control mode. It will be apparent that the transition levels of 2.0 volts and 2.5 volts are exemplary and different levels could be selected.

Still referring to FIG. 13 , in the very light loading region 10 and the light loading region 12, the flyback converter preferably operates in accordance with DCM. Under these loading conditions (e.g., in regions 10 and 12 of FIG. 13 ), the high side switch SW₂ can be operated in accordance with the first mode of operation of the high side switch SW₂ (as in FIG. 3 ), or in accordance with the second mode of operation (as in FIG. 4 ) or in accordance with both the first and second modes of operation of the high side switch SW₂ (the first and second modes being operative together as in FIG. 5 ) or in accordance with the third mode of operation (as in FIG. 6 ). Selection among the above-described modes of operation can be made, for example, experimentally according to which mode results in highest overall efficiency and which avoids overcharging the snubber capacitor C_(SN). For example, the above-described alternatives may assume “high line” conditions are present. However, under low line and light load conditions, it may be expected that the third mode (as in FIG. 9 ) or the combined first and second modes (as in FIG. 10 ) may be optimal. Under low line conditions and light load conditions, operation may be in accordance with the combined second and third modes (as in FIG. 11 ).

In an embodiment, under heavy loading conditions (e.g., in region 16 of FIG. 13 ), the switching power supply is preferably operated in accordance with CRM. Under heavy loading and “high line” conditions, the high side switch can, for example, be operated in accordance with the first mode of operation of the high side switch SW₂ (as in FIG. 7 ) or in accordance with the combined mode of operation (as in FIG. 8 ). Under heavy loading and “low line” conditions, the high side switch can, for example, be operated in accordance with the third mode of operation of the high side switch SW₂ (as in FIG. 12 ).

In an embodiment, under transitional loading conditions (as in region 14 of FIG. 13 ), where the loading level is between the light and heavy loading conditions, the switching power supply can be operated in DCM or CRM. Whether the switching power supply operates in DCM or CRM may depend, for example, upon whether operation just prior to entering the region 14 was DCM or CRM. More particularly, if the operation prior to entering the region 14 was DCM then DCM operation may continue in the region 14 until the error signal exceeds 2.5 volts. Conversely, if the operation prior to entering the region 14 was CRM then CRM operation may continue in the region 14 until the error signal falls below 2.0 volts. Also, under transitional loading conditions, the high side switch is preferably operated in accordance with the second mode of operation of the high side switch (as in FIG. 4 or 5 ). Similarly, the mode of operation in the transitional region 14 may remain the same as it was prior to entering the transitional region 14 until the load power crosses to the opposite side the transitional region 14. More particularly, the mode of operation present in region 16 may continue into region 14 until load power crosses into region 12. Conversely, the mode of operation present in region 12 may continue into region 14 until load power crosses into region 16.

The waveforms of FIGS. 3 through 12 apply equally to the current control and frequency control modes, though the time scale will change, dependent upon the mode of operation.

FIG. 14 illustrates a flyback converter 150 and control circuitry in accordance with an embodiment of the present invention. The flyback converter 150 of FIG. 2 is shown in FIG. 14 along with an embodiment of control circuitry and an embodiment of a second output stage. The flyback converter 150 and control circuitry of FIG. 14 are preferably configured to control the high side switch SW₂ and the low side switch SW₁. In particular, a “low driver” controller 152 generates a signal LOWOUT that controls (i.e. opens and closes) the low side switch SW₁. The low driver controller 152 can control the switch SW₁ using frequency control and/or peak current control in a feedback loop so as to regulate the output voltage V_(O), as described herein. The low driver controller 152 preferably also generates the signal READYHIGHON described herein. A “high driver” controller 154 generates a signal HIGHOUT that controls (i.e. opens and closes) the high side switch SW₂ based on the monitored levels of

and V_(IN), and based on the signal READYHIGHON, as described herein.

As shown in FIG. 14 , a resistive divider and photo-couple network 156 is coupled to the flyback converter 150 output and includes resistors R₁, R₂, and R₃, capacitor C₂, photo-diode P_(1A), and shunt regulator U₁. The photo-diode P_(1A) is optically coupled to phototransistor P_(1B). The phototransistor P_(1B) is coupled to a compensation resistor R₄ and capacitor C₃. A voltage signal V_(EAO) is generated across the compensation resistor R₄ and capacitor C₃. The signal V_(EAO) is representative of an error signal (a difference between the level of V_(O) and a desired level for V_(O)) and is also representative of a level of input power to the flyback converter 150. The signal V_(EAO) is electrically isolated from the output voltage V_(O) and is instead referenced to the ground level of the primary side of the transformer T₁.

The transformer T₁ can include a second secondary winding. As shown in FIG. 14 , a first terminal of the second secondary winding of the transformer T₁ is coupled to an anode of a diode D₂. A cathode of the diode D₂ is coupled to a first terminal of a capacitor C₄. A second terminal of the second secondary winding of the transformer T₁ is coupled to a second terminal of the capacitor C₄ and to the first ground node. A voltage V_(CC) is formed across the capacitor C₄ and can be used for powering control circuitry of the flyback converter 150. A resistive divider includes resistors R₅ and R₆ and generates a voltage signal ZCD that is representative of the level of V_(CC). The signal ZCD is also representative of the level of

.

As also shown in FIG. 14 , a first terminal of a capacitor

is coupled to the node between the high side switch SW₂ and the low side switch SW₁. A second terminal of the capacitor

is coupled to the first ground node. The capacitor

tends to smooth the

signal waveform. Additionally, a current sensing resistor R_(SENSE) is coupled between the second terminal of the transistor switch SW₁ and the first ground node. A current sensing signal I_(SENSE) is formed across the resistor R_(SENSE).

The low driver controller 152 receives as inputs the signals ZCD, I_(SENSE), V_(EAO) as well as an oscillator signal OSC and uses these signals to generate the signal LOWOUT for controlling the transistor switch SW₁ as explained herein. The signal V_(EAO) represents the load power and is used to regulate the output voltage in a feedback loop based on either peak current control or switching frequency control. The signal I_(SENSE) represents the current in the transformer T₁ and is used to the control peak current in the transformer primary winding during switching. The oscillator signal OSC is used for controlling the timing of switching. The signal ZCD is representative of the level of

and is used to turn on the switch SW₁.

The low driver controller 152 generates a differential signal READYHIGHON which is used by the high driver controller 154 for controlling the transistor switch SW₂ as explained herein. The signal READYHIGHON informs the high driver controller 154 that it can (i.e. has permission to) turn on the switch SW₂, though the high driver controller 154 generally determines the timing of turning on the switch SW₂. The signal READYHIGHON is preferably a differential signal because the low driver controller 152 and the high driver controller 154 preferably have different ground reference nodes. In particular, the low driver controller 152 is referenced to the first ground node, whereas, the high driver controller 154 preferably uses the voltage

as its ground reference.

As also shown in FIG. 14 , a first terminal of a first high-voltage resistor R_(HV1) is coupled to the second terminal of the capacitor C_(SN). A second terminal of the resistor R_(HV1) is coupled to the high driver controller 154. This provides the high driver controller 154 with a signal CS that is representative of the voltage V_(CSN). A first terminal of a second high-voltage resistor R_(HV2) is coupled to the input voltage V_(IN). A second terminal of the resistor R_(HV2) is coupled to the high driver controller 154. A capacitor C_(RVIN) is preferably coupled in parallel with the resistor R_(HV2). This provides the high driver controller 154 with a signal R_(VIN) that is representative of the voltage V_(IN). The capacitor C_(RVIN) helps in smoothing the signal R_(VIN) particularly at light loads. The voltage

signal is also coupled to the high driver controller 154. The high driver controller 154 uses the signals R_(VIN), CS,

, and READYHIGHON to generate the signal HIGHOUT that controls (i.e. opens and closes) the switch SW₂ as explained herein. For example, when

is greater than V_(IN) and CS is substantially equal to

, the high driver controller 154 turns on the switch SW₂. The switch SW₂ generally stays on until

is substantially equal to V_(IN) and then the switch SW₂ is turned off though, as explained herein, the time instant at which the switch SW₂ is turned on and off can also be dependent upon other factors, such as the level of the input voltage V_(IN). The switch SW₂ can also be turned off when the signal READYHIGHON is deactivated.

Also shown in FIG. 14 , the voltage V_(CC) can be used as a power supply for powering elements of the low driver controller 152. A voltage V_(BOOT) can be used as a power supply for powering elements of the high driver controller 154. The voltage V_(BOOT) can be obtained by drawing current from V_(CC), for example, via a diode which then charges a capacitor C_(VBOOT). The voltage V_(BOOT) can also be used to indicate a level of loading of the power converter 150 as explained herein.

An exemplary switching cycle is performed as follows. The low side switch SW₁ is turned on. Then, once the peak current in the primary winding of the transformer T₁ is reached, as indicated by the current sensing signal I_(SENSE), the low side switch SW₁ is turned off. The peak current depends on the level of V_(EAO): (1) when V_(EAO) is less than a threshold (e.g. 2.5 volts), then the flyback converter is in frequency control mode and the peak current is essentially a fixed value (though the peak current is preferably gradually reduced as VEAO falls in order to increase efficiency and inhibit audible noise in burst mode); (2) when V_(EAO) is greater than the threshold (e.g. 2.5 volts), then the flyback converter is in current control mode and the peak current depends on V_(EAO) (and the switching frequency is clamped). Once the low side switch SW₁ turns off, the voltage

flys up, eventually reaching a level above the input voltage V_(IN). The low side driver 152 then activates sending the READYHIGHON signal to the high side driver 154. The READYHIGHON signal is activated at a time that depends upon the switching frequency and other factors. After receiving the READYHIGHON signal, the high side driver 154 determines that

is greater than V_(IN) by an appropriate margin and, in response, the high side driver 154 turns on the high side switch SW₂. The high side switch SW₂ remains on for a period of time, e.g., until the level of

falls to the level of V_(IN), or until the signal READYHIGHON is deactivated, at which time, high side driver 154 turns off the high side switch SW₂. The time instant at which the high side driver 154 turns off the high side switch SW₂ can also be adjusted according to a level of V_(IN) as explained herein. More particularly, the high side switch SW₂ can be opened before

falls to the level of V_(IN) by an amount that depends on the level of V_(IN). When the level of

falls to zero, the low side switch SW₁ can be turned on again.

For the third mode of operation (e.g., FIGS. 6, 8, 9 and 11-12 ), the READYHIGHON signal can serve as the pulse signal that the high driver 154 uses to open the high side switch SW₂. For example, the high driver 154 can be configured to open the switch SW₂ based only on the signal READYHIGHON without reference to, or monitoring of, the level of

. In this mode, the high side switch SW₂ can be held open for a pre-determined time period or for an adjustable time period. For example, the high side switch SW₂ can be closed at a time that depends upon a monitored level of R_(VIN) (and therefore the level of V_(IN)). In this case, the duration of the pulse signal is dependent upon the level of V_(IN). Additionally, the duration of the pulse signal can be adjusted by adjusting the resistance value of the resistor R_(HV2). In this “third” mode of operation, the low driver controller 152 may generate the READYHIGHON signal while the high driver controller 154 may not use the level of

to control the opening of the high side switch SW₂. Therefore, it is the low driver controller 152 that controls the timing of opening the switch SW₂, rather than the high driver controller 154. The low driver controller 152 does this by initiating the READYHIGHON pulse signal that the high driver 154 then uses to open the high side switch SW₂. This indirect control of the high side switch SW₂ may result in non-ZVS operation but with improved efficiency.

The alternative of the third mode of operation of the high side switch, in which the high side switch SW₂ is held off while the body diode of the high side switching transistor SW₂ still performs rectification, can be implemented similarly by the high driver 154 being configured to open the switch SW₂ only when the signal READYHIGHON is active (i.e. a logic high voltage). However, in this alternative third mode of operation, the low driver 152 can withhold the READYHIGHON signal, thereby, causing the high driver 154 to hold the high side switch SW₂ off. In this case, the body diode of the high side switch SW₂ performs rectification passively. This alternative third mode operation may therefore also be initiated by the low driver controller 152.

FIG. 15 illustrates a controller integrated circuit (IC) for a DC-to-DC converter in accordance with an embodiment of the present invention. In a preferred embodiment, the IC controller is implemented as an IC package 200 that includes the low driver controller 152 as a first monolithic IC chip and the high driver 154 as a second monolithic IC chip, both included in the same 18-pin IC package. In an embodiment, the switch SW₂ is integrated into the high driver controller 154 IC chip. Also, in an embodiment, the resistors R_(HV1) and R_(HV2) is included in the IC package. One or both of the resistors R_(HV1) and R_(HV2) can be integrated into the high driver controller 154 IC chip. Further, the resistor R_(HV2) can be partially integrated into the high driver controller 154 IC chip. As explained herein, each of the two IC chips has a different ground reference. Communication between the two chips is via the differential signal READYHIGHON.

FIG. 15 shows signals assigned to each of the 18 pins:

Pin 1 CS Pin 2 N/C Pin 3 V_(IN) Pin 4 N/C Pin 5 ZCD Pin 6 OTP Pin 7 OCS Pin 8 RESET Pin 9 V_(EAO) Pin 10 I_(SENSE) Pin 11 V_(SSD) Pin 12 LOWOUT Pin 13 V_(CC) Pin 14 N/C Pin 15 R_(VIN) Pin 16 V_(BOOT) Pin 17 HIGHOUT Pin 18 V 

Pin 2, pin 4, and pin 14 are not used and are labeled “N/C” or “no connection.” A diode is connected between pin 13 and pin 16. OTP can be an over-temperature protection pin that provides a current to an external thermistor, the voltage on which can then be compared to a reference, such as 1.0 volt, to detect an over-temperature condition. V_(SSD) is a ground pin. A reset pin RESET can be used to reset the ICs of the package after entering a protection mode. The reset can be accomplished by pulling the RESET pin to a voltage that is less than a reference voltage such as 2.5 volts.

FIG. 16 illustrates control circuitry of the high driver 154 in accordance with an embodiment of the present invention. A signal R_(VIN), which represents the input supply voltage V_(IN), is compared by a comparator 158 to a pair of reference currents I₁ and I₂ which are generated by current sources 160 and 162, respectively. The current sources 160 and 162 are coupled to

as a reference voltage. In an embodiment, the current I₁ is set to 2 μA (2 microamps) while the current I₂ is set to 60 μA (60 microamps). The signals at the input of the comparator 158 may additionally be adjusted, for example, in order to compensate for signal path delays during high-frequency operation.

The output of the comparator 158 is logic signal I_(LIMIT). I_(LIMIT) is a first logic level when the level of current received by the comparator 158 from R_(VIN) is between the levels of I₁ and I₂ and, otherwise, I_(LIMIT) is a second logic level. The signal I_(LIMIT) is used to control the timing of turning on the high side switch SW₂. More particularly, the R_(VIN) current level must be between the levels of I₁ and I₂ in order for the high side switch SW₂ to be turned on during a switching cycle (and the signal READYHIGHON must be activated). The comparator 158 is therefore a window comparator whose output indicates whether or not the R_(VIN) current signal is between the levels of I₁ and I₂. The comparator 158 determines that

is greater than V_(IN) by an appropriate margin and, in response to this determination, the high side driver 154 turns on the high side switch SW₂. Thus, the window of comparison implemented by comparator 158 determines the timing for turning on SW₂ (shown at time t₂ in FIGS. 3 and 4 ). Providing a window for comparison helps to counter effects of noise associated with high rates of signal change and accommodates parasitic capacitance of the R_(VIN) resistor R_(VH2) (FIG. 2 ) and avoids changing the logic level of ILIMIT at the moments when V_(IN) and

are crossing each other (when one is rising and the other is falling and vice-versa).

The output of the comparator 158 is inverted by an inverter 164 to form a logic signal I_(LIMIT)-bar. The signal I_(LIMIT)-bar is coupled to an input of a NAND gate 166 via logic OR gate 165, to an inverted set input S-bar of a flip-flop FF₁, to an input of a NAND gate 168 and to an inverted set input S-bar of a flip-flop FF₂. The signal I_(LIMIT) is coupled to an inverted set input S-bar to a flip-flop FF₃. An output Q of the flip-flip FF₃ is coupled to an input of a one-shot circuit 170. An inverted output of the one-shot circuit 170 is coupled to an inverted reset input R-bar to the flip-flip FF₂. An inverted output Q-bar of the flip-flop FF₂ is coupled to an input of the NAND gate 166.

The signal READYHIGHON is coupled to an input of the NAND gate 166, to a first inverted reset input R₁-bar to the flip-flop FF₁, to an input of a delay 172 and to an input of the NAND gate 168. An inverted under-voltage lockout signal U_(VLO)-bar is coupled to a first inverted reset input R₁-bar to the flip-flop FF₃ and to second inverted reset input R_(s)-bar to the flip-flop FF₁. An output Q of the flip-flop FF₁ is coupled to an input of the NAND gate 168. An output of the delay 168 is coupled to an input of the NAND gate 168.

An output of the NAND gate 166 is coupled to a first inverted set input S₁-bar of a flip-flop FF₄. An output of the NAND gate 168 is coupled to a second inverted set input S₂-bar of the flip-flop FF₄. An output Q of the flip-flop FF₄ is coupled to an input of an AND gate 174. An inverted output of the gate 174 is coupled to a second inverted reset input to the flip-flop FF₃. A non-inverted output of the gate 174 forms the signal HIGHOUT. The generated signal HIGHOUT is used to control the switch SW₂.

The signal R_(VIN) is also compared by a comparator 176 to a current I₃. The current I₃ is generated by an adjustable current source 178. The current source 178 is coupled to

as a reference voltage. The current I₃ is adjusted based on the level of V_(IN). The output of the comparator 176 is coupled to a first input of a NAND gate 180. The signal HIGHOUT is coupled to a second input to the NAND gate 180. An output of the NAND gate 180 is coupled to a first input of a NAND gate 182. The inverted under-voltage lockout signal U_(VLO)-bar is coupled to a second input of the NAND gate 182. An output of the NAND gate 182 is coupled to an inverted reset input R-bar to the flip-flop FF₄ and to a first input to a sample/logic circuit 184.

The signal R_(VIN) is coupled to a second input of the sample/logic circuit 184. The NAND gate 182 generates a signal High Side Off, which is used by the sample/logic circuit 184 for sampling V_(IN). More particularly, each time that the high side switch SW₂ is turned off (i.e. opened), the sample circuit preferably waits a delay period of approximately 250 to 300 nanoseconds and then samples the level of V_(IN). Therefore, the samples are taken at approximately when

is equal to zero volts. Successive samples of V_(IN) can be averaged to ensure that the monitored level of V_(IN) does not change rapidly. The sample/logic circuit 184 generates a signal ˜V_(IN) (steady-state) which is representative of the level of V_(IN). This signal is used for generating the current I₃ which is also representative of the level of V_(IN).

Elements of FIG. 16 detect the level of

for controlling switching of SW₂ as shown in FIGS. 3-5, 7, 8 and 10 . The signal READYHIGHON informs the high driver controller 154 that it can (i.e. has permission to) turn on the switch SW₂. The high driver controller 154 then determines the timing of turning on the switch SW₂: when

is reaches the level of V_(IN) (as indicated by the signal I_(LIMIT)) the high driver controller 154 turns on the switch SW₂.

The flip-flop FF₁ and the delay block 172 are used to delay turning on the switch SW₂ so as to avoid turning on the switch prematurely. The U_(VLO) signal inhibits switching in case of an under-voltage condition.

The switch SW₂ stays on until the switch SW₂ is turned off based on comparison performed by comparator 176. More particularly, the switch SW₂ is turned off (opened) when V_(IN) is substantially equal to

as offset by an amount indicated by the level of ˜V_(IN) (steady-state). The comparator 176 preferably performs its comparison with hysteresis so as to avoid multiple changes in the level of its output at crossings of the input signal levels of the comparator 176.

As shown in FIG. 16 , the voltage supply V_(BOOT) may be coupled to a third input to the sample/logic circuit 184. This can be used for determining whether the power converter 150 is under light or heavy loading conditions. For example, V_(BOOT) may be regulated to approximately 15 volts DC. When V_(BOOT) rises above a threshold level of 15 volts, this indicates light loading. Conversely, when V_(BOOT) falls below the threshold level of 15 volts, this indicates that the loading is no longer light. In accordance with an embodiment of the present invention, when the converter 150 is under light loading conditions, the function of turning off the switch SW₂ based on the level of V_(IN) can be disabled. More particularly, when the level of V_(BOOT) is above the threshold, the adjustment to the current I₃ based on the level of ˜V_(IN) (steady-state) can be made equal to zero. When this occurs, the comparator 176 can compare the level of V_(IN) (as represented by R_(VIN)) to the level of

in order to determine when to open the switch SW₂. Under these conditions, the switch SW₂ can be opened under ZVS. Additionally, when the level of V_(IN) is at a maximum threshold (e.g. 380 volts DC), the adjustment to the current I₃ based on the level of ˜V_(IN) (steady-state) can also be equal to zero. However, when the level of V_(BOOT) is above the threshold (and the level of V_(IN) is below its maximum threshold), the level of the current I₃ can be representative of the level of V_(IN) so that the switch SW₂ is opened based in part on the monitored level of V_(IN) (i.e. ˜V_(IN) (steady-state)).

The sample/logic circuit 184 may also be used to determine when the high side driver controller 154 enters the third mode of operation. More particularly, the sample/logic circuit 184 may use the R_(VIN) and V_(BOOT) signals to make this determination since those signals are representative of the input voltage and load power, respectively. A signal HIGH SIDE PULSE ENABLE is activated to indicate that the high side driver controller 154 has entered the third mode of operation. This signal is applied to the logic OR gate 165 which overrides the determination by the high driver controller 154 of the timing for turning on the switch SW₂. Instead, the switch SW₂ is directed to turn on and/or off in response to receipt of the READYHIGHON signal. It will be apparent that the high side driver 154 can determine whether to enter the third mode in another manner. This can include, for example, monitoring the input voltage and load power in another manner and/or bypassing the

sensing for turning on the switch SW₂ in another manner. Alternatively, the signal HIGH SIDE PULSE ENABLE need not be generated. In this case, the logic OR gate 165 can be omitted and the signal I_(LIMIT)-bar can be coupled to the logic NAND gate 166. In this case, the signal I_(LIMIT)-bar and the signal READYHIGHON must both be active in order for the high side switch to be closed.

FIG. 17 illustrates control circuitry of the low driver 152 in accordance with an embodiment of the present invention. As shown in FIG. 17 , the low driver 152 includes a current control section 186, a frequency control section 188, a timer section 190, switching logic 192 and a switch driver 194.

Within the current control section 186 of the low driver 152, the signal V_(EAO) is coupled to a first input to a comparator 196. A second input to the comparator 196 receives a first reference voltage (e.g. 2.5 volts) while a third input to the comparator 196 receives a second reference voltage (e.g. 2.0 volts). The comparator 196 generates a signal “V_(EAO)>2.5v−bar” by comparing the signal V_(EAO) to the first and second reference voltages; the signal “V_(EAO)>2.5v−bar” is activated when V_(EAO) rises above the first reference and is deactivated when the signal V_(EAO) falls below the second reference. Thus, the comparator 196 performs its comparison with hysteresis. The comparator 196 determines whether low driver controller 152 performs switching based on peak current control or based on frequency control. When VEAO rises above 2.5 volts, switching is by peak current control; when V_(EAO) falls below 2.0 volts, switching is by frequency control. Thus, the logic level of “V_(EAO)>2.5v−bar” determines whether the switching is based on peak current control or frequency control.

The signal I_(SENSE) is coupled an input to a first amplifier 198 and to an input to a second amplifier 200. The amplifier 198 can have, for example, a gain of 15, while the amplifier 200 can have a gain of, for example 7.5. The output of the amplifier 198 is coupled to a first input to a comparator 202 via a switch S₁. The output of the amplifier 200 is coupled to the first input of the comparator via a switch S₂. The signal V_(EAO) is coupled to a second input to the comparator CMP4. The signal “VEAO>2.5v−bar” is coupled to control the switch S₂ and to control the switch S₁ via an inverter 204. Thus, one of the switches S₁ are S₂ is closed while the other is opened dependent upon the signal “V_(EAO)>2.5v−bar”. Accordingly, the outputs of the amplifiers 198 and 200 are selectively coupled to the first input of a comparator 202 dependent upon the level of V_(EAO). An output of the comparator 202 is coupled to an input to switching logic 192.

In accordance with current control, the amplifier 198 having higher gain is active so as to magnify the effect of I_(SENSE) in comparison to VEAO by comparator 202. In accordance with frequency control, the amplifier 200 is active which employs lower gain so as to reduce the effect of I_(SENSE) in the comparison which causes the frequency control section 188 to primarily control switching.

Within the frequency control section 188 of the low driver 152, the signal I_(SENSE) is coupled to an input to an amplifier 206. The amplifier 206 can have, for example, a gain of 7.5. An output of the amplifier 206 is coupled to a first input to a comparator 208. A second input to the comparator is coupled to a reference voltage, which can be, for example, approximately 2.5 volts. An output of the comparator 208 is coupled to an input to switching logic 178.

The current control section 186 and the frequency control section 188 control the timing of turning off the low side switch SW₁ within each switching cycle via switching logic 192.

The timer section 190 of the low driver 152 controls switching frequency as well as the timing for turning on the low side switch SW₁ for each switching cycle. The timer section 190 also generates the signal HON which is used to generate the signal READYHIGHON (see FIG. 18 ). The signal READYHIGHON is used by the high driver controller 154 to control the switch SW₂ for the first and second modes of operation of the high side switch SW₂ and is also used to generate the pulse signal for the third mode of operation of the high side switch SW₂. Within the timer section 190, the signal V_(EAO) is coupled to a first input to an oscillator 210. The signal “V_(EAO)>2.5v−bar” from the comparator 196 is coupled to a second input of the oscillator 210. The oscillator 210 generates a periodic ramp signal that is coupled to an input of timer/logic block 212.

The timer/logic block 212 generates a logic signal HON and a logic signal ONSET which are coupled to the switching logic 192. The signal HON is used by the low driver controller 152 (FIG. 14 ) to generate the signal READYHIGHON for the high driver controller 154. For peak current control, the signal HON is generated at fixed intervals. The signal ONSET is used to turn on the low side switch SW₁. For example, a timer of 3.33 microseconds can be reset for each switching cycle; 500 nanoseconds prior to expiration of the timer, the signal HON is activated. And, upon expiration of the timer, and once ZCD is greater than zero, then the signal ONSET can be activated. Once ONSET is activated, then the low side switch SW₁ can be closed upon a valley in signal ZCD (since ZCD represents

) so as to operate the switch SW₁ under zero volt switching (ZVS) conditions. The 500 nanosecond difference ensures that HON is activated prior to ONSET.

The signal ZCD is referenced to the same ground level as the low side driver controller 152. The signal ZCD is also representative of the level of

. Thus, signal ZCD is used by the low side driver controller 152 as a proxy for

in order to operate the switch SW₁ under ZVS conditions.

For current control, rather than a fixed timer interval of 3.33 microseconds, for example, the timer interval is varied dependent upon the level of V_(EAO). Thus, the timer interval affects the switching frequency for regulating the output voltage in a feedback loop.

The timer interval of 3.33 microseconds corresponds to a switching frequency for peak current control of 300 kHz. In an embodiment, the switching frequency f_(clamp) can be clamped at 500 kHz, 300 kHz, 145 kHz, or some other selected frequency by appropriate selection of timing components.

An output of the switching logic 192 is coupled to a driver 194. The driver 194 generates the signal LOWOUT.

During a switching cycle of the switches SW₁ and SW₂, the level of

rises above the level of V_(IN) immediately upon opening of the low side switch SW₁. Under light load conditions, energy stored in the primary side of the transformer T₁ is not as effectively transferred to its secondary side. The energy stored in the snubber capacitor C_(SN) can therefore increase with each switching cycle so that it becomes overcharged with energy and so that the level of

can ring to an excessively high level upon opening of the low side switch SW₁. These effects of a light load condition can be mitigated by turning on the high side switch SW₂ earlier in the switching cycle. Thus, in accordance with an embodiment of the present invention, the high side switch SW₂ is turned on earlier in the switching cycle under certain light load conditions than it would otherwise be turned on. Similarly, when the input supply voltage V_(IN) is at a high level, this can also result in overcharging the snubber capacitor C_(SN). This can also be mitigated by turning on the high side switch SW₂ earlier in the switching cycle. Using frequency control to control switching when the input voltage V_(IN) is at a high level, rather than current control, can also help to mitigate these issues. Thus, in accordance with an embodiment of the present invention, frequency control is used under certain high input voltage conditions.

In an embodiment, a comparator 214 is included in the current control section 186 of FIG. 17 which compares the level of the signal R_(VIN) (which is representative of the voltage V_(IN)) to a reference. As shown in FIG. 17 , the signal R_(VIN) can be stepped down by a voltage divider that includes resistors R₁₁ and R₁₂. The stepped down voltage is representative of the level of V_(IN) and is compared to a reference voltage V_(REF2). When this comparison indicates that the level of V_(IN) is greater than a threshold (e.g., 226 volts DC), then the output of the comparator 214 disables the comparator 196. As a result, the switch S₁ is open and the switch S₂ is closed so that the amplifier 200 is active. This causes the frequency control section 188 of the flyback converter control circuitry to primarily control switching, rather than the current control section 186. Thus, the controller of the flyback converter operates in the frequency control mode upon detection of a supply voltage condition (e.g. when V_(IN) is greater than the threshold of 226 volts DC).

In an embodiment, the timer section 190 in FIG. 17 includes a comparator 216. The comparator 216 is configured to compare the stepped down voltage that is representative of the level of V_(IN) (at the node between resistors R₁₁ and R₁₂) to a reference voltage V_(REF3). When this comparison indicates that the level of V_(IN) is lower than a threshold, then the output of the comparator 216 generates a signal LOWLINE which is coupled to the time 212. In response, the timer/logic 212 can generate the pulse signal discussed herein which is then used to generate the signals HON which is used by the low driver 152 to generate READYHIGHON in accordance with the third mode of operation of the high side switch SW₂.

FIG. 18 illustrates a differential signal converter 300 for use in control circuitry for a flyback converter in accordance with an embodiment of the present invention. The differential signal converter 300 converts the single-ended signal HON to a differential logic signal READYHIGHON. The logic signal HON is coupled to an input of a first inverter 302. An output of the first inverter 302 is coupled to an input to a second inverter 304 and to control a MOSFET M₅. An output of the second inverter 304 is coupled to control a MOSFET M₆. A MOSFET M₇ and a current source 306 are coupled in series with the MOSFET M₅. A MOSFET M₈ and a current source 308 are coupled in series with the MOSFET M₆. A current source 310 is coupled in series with MOSFET M₉ and MOSFET M₁₀. A reference current passes through the MOSFETS M₉ and M₁₀. The signal HON activates one of the MOSFETS M₅ or M₇ dependent upon the level of HON. The reference current is mirrored in the MOSFET M₈ or in the MOSFET M₇ dependent upon which of the MOSFETS M₅ or M₇ is active. The state of the differential signal READYHIGHON is dependent upon which of the MOSFETS M₅ or M₇ is active. Thus, the converter 300 converts the logic signal HON to the differential logic signal READYHIGHON.

FIG. 19 illustrates an oscillator for use in control circuitry for a flyback converter in accordance with an embodiment of the present invention. FIG. 19 shows additional details of the oscillator 190 of FIG. 15 . As shown in FIG. 19 , the signal V_(EAO) is coupled to a first input to an amplifier AMP₁ via a switch S₃. A reference voltage of, for example, 2.5 volts, is coupled to a second input of the amplifier AMP₁ via a switch S₄. A third input to the amplifier AMP₁ is coupled to a first terminal of an adjustable resistor R_(OSC1) and to a first terminal of a resistor R_(OSC2). An output of the amplifier AMP₁ is coupled to a control terminal of MOSFET M₁₁. An output terminal of the MOSFET M₁₁ is coupled to the first terminal of the resistor R_(OSC1) and to the first terminal of the resistor R_(OSC2). A second terminal of the resistor R_(OSC1) is coupled to a ground node via a switch S₅. A second terminal of the resistor R_(OSC2) is coupled to the ground node via a switch S₆.

A supply voltage VCC is coupled to an input terminal of a MOSFET M₁₂ and to an input terminal of a MOSFET M₁₃. An output terminal of the MOSFET M₁₂ is coupled to a control terminal of the MOSFET M₁₂, to a control terminal of the MOSFET M₁₃ and to an input terminal of the MOSFET M₁₁. An output terminal of the MOSFET M₁₃ is coupled to a first terminal of an adjustable capacitor C_(T), to a first input terminal (inverting) to a comparator CMP₆ and to a first input terminal (non-inverting) to a comparator CMP₇. A second terminal of the adjustable capacitor C_(T) is coupled to the ground node. A second input terminal to the comparator CMP₆ is coupled to a reference voltage Vrefh. A second input terminal to the comparator CMP ₇ is coupled to a reference voltage Vrefl. An output of the comparator CMP₆ is coupled to an inverted set input S-bar of a flip-flop FF₅. An output of the comparator CMP ₇ is coupled to an inverted reset input R-bar of the flip-flop FF₅. An output Q of the flip-flop FF₅ is coupled to control a switch S₇. The switch S₇ is coupled to across the capacitor C_(T).

The switches S₃ and S₅ are controlled by the signal “V_(EAO)>2.5v” while the switches S₄ and S₆ are controlled by the logic signal “V_(EAO)>2.5v−bar”. Thus, when V_(EAO) is greater than the 2.5 volt threshold, the switches S₃ and S₅ are closed and the switches S₄ and S₆ are open; when V_(EAO) is below the 2.0 volt threshold, the switches S₄ and S₆ are closed and the switches S₃ and S₅ are open. As explained herein, the signals “V_(EAO)>2.5v” and its inverse “V_(EAO)>2.5v−bar” are generated with hysteresis.

The oscillator 190 generates a periodic ramp signal RTCT across the capacitor C_(T). The transistors M₁₂ and M₁₃ form a current mirror such that the current through the transistor M₁₃ charges the capacitor C_(T). When the voltage across the capacitor C_(T) reaches Vrefh, the capacitor C_(T) is discharged by closing the switch S₇ until the voltage across the capacitor C_(T) falls below Vrefl. The switch S₇ is then opened.

The frequency of the ramp signal RTCT is changed dependent upon of the state of the logic signal “V_(EAO)>2.5v”. More particularly, when V_(EAO) is less than 2.0 volts (the signal “V_(EAO)>2.5v” is a logic “0”), the flyback converter operates in the frequency control mode in which the switching frequency and is dependent upon the level of V_(EAO). This is accomplished by closing the switch S₃ so that V_(EAO) is coupled to the amplifier AMP₁ which turns on the MOSFET M₁₁ in relation to the level of V_(EAO). The level of current in the current mirror of MOSFET M₁₂ and M₁₃ is, therefore, affected by the level of V_(EAO) which, in turn, affects the rate of charging the capacitor C_(T) and the frequency of the ramp signal RTCT. The frequency of the ramp signal RTCT is the same as the switching frequency of the flyback converter. Thus, in this frequency control mode, the switching frequency is controlled in a feedback loop to regulate the output voltage where the switching frequency is dependent upon V_(EAO).

The frequency control mode continues unless V_(EAO) rises above 2.5 volts. When V_(EAO) rises above 2.5 volts and the signal “V_(EAO)>2.5v” becomes a logic “1” then the switch S₃ is opened and the switch S₄ is closed which couples a fixed reference voltage to the input of the amplifier AMP₁ so that the current that charges the capacitor C_(T) is essentially constant. This causes the switching frequency for the flyback converter to be essentially constant; in this mode, current is controlled in feedback loop to regulated the output voltage.

The frequency of the ramp signal RTCT and, thus, the switching frequency of the flyback converter is dependent upon the value of C_(T) as well as the resistor R_(OSC1) and R_(OSC2). In the current control mode, the switch S₆ is closed so that the resistor R_(OSC2) affects the switching frequency whereas the switch S₅ is opened so that the resistor R_(OSC1) does not affect the switching frequency. In the frequency control mode, the switch S₆ opened so that the resistor R_(OSC2) no longer affects the switching frequency and the switch S₅ is closed so that the resistor R_(OSC1) does affect the switching frequency.

The values of C_(T), R_(OSC1) and R_(OSC2) are selected so as to appropriately set the nominal switching frequency in the frequency control mode, as well as the essentially fixed switching frequency in the current control mode. Additionally, the values of the resistor R_(OSC1) and the capacitor C_(T) can preferably be fine-tuned, e.g. by laser or fuse trimming in order to ensure that there is a smooth transition between the frequency control and current control modes. For this purpose, the resistor ROSC1 is preferably incorporated into the IC package shown in FIG. 15 .

Component selection for the oscillator can include first selecting a value for the resistor R_(OSC2), which sets the clamping frequency f_(clamp). Then, the capacitor C_(T), which is preferably internal to the low driver controller IC 152, is trimmed to fine-tune the clamping frequency. Finally, the internal resistor ROSC1, which is also preferably internal to the low driver controller IC 152, is trimmed to fine-tune the switching frequency at the transition between current control and frequency control modes of operation.

FIG. 20 illustrates a comparator for use in control circuitry for a flyback converter in accordance with an embodiment of the present invention. The comparator of FIG. 20 can be used in place of the comparator CMP5 shown in the frequency mode control section 174 of FIG. 15 . As shown in FIGS. 10 and 14 , the comparator accepts as input the signal I_(SENSE)×7.5 which is compared to a reference voltage of 2.5 volts for generating the signal OFF. The signal OFF is used to turn off the main switch SW₁. The comparator of FIG. 20 additionally accepts as input the signal V_(EAO). The signal V_(EAO) reduces the effective level of the reference voltage thereby generating the signal OFF sooner and therefore reducing the switching frequency. This is useful to reduce switching noise in burst mode.

FIG. 21 illustrates an embodiment of a timer/logic in accordance with an embodiment of the present invention. The timer 212 of FIG. 17 can be replaced with timer/logic 218 of FIG. 21 . As shown in FIG. 21 , the timer/logic 218 accepts as input a “Load Condition” signal and an “Input Condition” signal (which corresponds to the LOWLINE signal described herein), in addition to the oscillator signal OSC. The timer/logic 218 uses these signals to generate the signals HON and ONSET described herein. The Load Condition and Input Condition signals are binary (i.e. each having two possible states such as true/false or zero/one) and indicate when the current loading is light and when the input voltage V_(IN) is high, respectively. Because the signal V_(EAO) is indicative of loading, the Load Condition signal can be equivalent to the logic signal “V_(EAO)>2.5v−bar”.

Similarly to the timer 212 of FIG. 17 , the timer 218 of FIG. 21 , generates a logic signal HON and a logic signal ONSET, which are coupled to the switching logic 178. The signal HON is used to generate the signal READYHIGHON for the high driver controller 154. For frequency control, the signals HON and ONSET are generated at intervals that depend on the switching frequency as determined by the frequency of the oscillator signal OSC. For example, a timer having a variable time duration can be reset for each switching cycle. This time duration will change dependent upon the switch frequency. The time duration for a particular frequency can be, for example, 3.33 microseconds. At an instant prior to expiration of the timer, the signal HON is activated. The amount of time prior to expiration of the timer that the activation of HON occurs is also dependent upon the switching frequency but can also be dependent upon the levels of Load Condition and Input Condition signals in order to turn on the high side switch SW₂ earlier in the switching cycle. For example, when the switching frequency results in the timer duration being 3.33 microseconds, the activation can occur 500 nanoseconds prior to expiration of the timer under normal conditions. However, if one or both of the Load Condition and Input Condition signals indicates a light load condition or a high input voltage condition, then the time duration may be adjusted (e.g. increased to 1000 nanoseconds or 1.0 microsecond) in order to activate the signal HON earlier in the switching cycle and therefore to also turn on the switch SW₂ earlier in the switching cycle.

As explained herein, the signal HON can be used to generate the signal READYHIGHON which is used by the high driver controller 154 (FIG. 14 ) to control the mode of operation of the high side switch SW₂. The low driver controller 152 therefore can determine the mode of operation (e.g.,“first” mode, “second” mode, “third” mode or a combination thereof, as described herein) of the high side switch SW₂. More particularly, the timer/logic 218 of the low driver controller 152 can determine the mode of operation of the high side switch SW₂.

The foregoing detailed description of the present invention is provided for the purposes of illustration and is not intended to be exhaustive or to limit the invention to the embodiments disclosed. Accordingly, the scope of the present invention is defined by the appended claims. 

What is claimed is:
 1. A controller configured to control a power converter, the power converter having a transformer, a low side switch for drawing current from a supply voltage through a primary winding of the transformer and a high side switch for coupling the primary winding of the transformer to a snubber capacitor, wherein the controller is configured to generate drive signals that control the opening and closing of the low side switch once during each of a series of switching cycles to form a regulated output voltage, and wherein the controller is configured to selectively control the high side switch according to: a mode of operation in which the high side switch is closed and then opened once during each of the series of switching cycles; and a mode of operation in which the high side switch is closed and then opened two times during each of the series of switching cycles.
 2. The controller according to claim 1, wherein the controller is configured to control a flyback converter that performs DC-to-DC power conversion.
 3. The controller according to claim 1, wherein the controller is configured to control the power converter so that output voltage is formed from a current induced in a secondary winding of the transformer and wherein the output voltage is regulated in a feedback loop.
 4. The controller according to claim 1, wherein the controller is configured to select between the mode of operation in which the high side switch is closed and then opened once during each of the series of switching cycles and the mode of operation in which the high side switch is closed then opened two times during each of the series of switching cycles according to a level of power delivered to a load.
 5. The controller according to claim 1, wherein the controller is configured to select the mode of operation in which the high side switch is closed then opened two times during each of the series of switching cycles when the level of power delivered to the load is light.
 6. The controller according to claim 1, wherein the controller is configured to select between the mode of operation in which the high side switch is closed and then opened once during each of the series of switching cycles and the mode of operation in which the high side switch is closed then opened two times during each of the series of switching cycles according to a level of line voltage available as input to the power converter.
 7. A switching power supply comprising: the controller according to claim 1; and a power converter comprising the transformer, the low side switch, the high side switch and the snubber capacitor.
 8. A controller configured to control a power converter, the power converter having a transformer, a low side switch for drawing current from a supply voltage through a primary winding of the transformer and a high side switch for coupling the primary winding of the transformer to a snubber capacitor, wherein the controller is configured to generate drive signals that control the opening and closing of the low side switch once during each of a series of switching cycles to form a regulated output voltage, and wherein the controller is configured to selectively control the high side switch according to: a mode of operation in which the high side switch is closed and then opened once during each of the series of switching cycles; and a mode of operation in which the high side switch is closed and then opened two times during each of the series of switching cycles, wherein the controller is configured to monitor a first voltage generated at a node between the low side switch and the high side switch for determining time instants for closing and opening the high side switch.
 9. The controller according to claim 8, wherein when the mode of operation in which the high side switch is closed and then opened once during each of the series of switching cycles is selected, the high side switch is closed when the level of the first voltage is falling.
 10. The controller according to claim 8, wherein when the mode of operation in which the high side switch is closed and then opened once during each of the series of switching cycles is selected, the high side switch is closed when the level of the first voltage is rising.
 11. The controller according to claim 8, wherein the high side switch is closed in accordance with zero volt switching.
 12. A controller configured to control a power converter, the power converter having a transformer, a low side switch for drawing current from a supply voltage through a primary winding of the transformer and a high side switch for coupling the primary winding of the transformer to a snubber capacitor, wherein a first voltage is generated at a node between the low side switch and the high side switch, and wherein the controller is configured to generate a drive signal that controls the opening and closing of the low side switch to form a regulated output voltage, and wherein the controller is configured to selectively operate in a mode in which the high side switch is closed during each of a series of switching cycles based on a comparison of the supply voltage to the first voltage and in a mode in which the high side switch is held open during each of a series of switching cycles thereby a current through the high side switch is passively rectified by a body diode of the high side switch, and wherein the controller is configured to select between the modes of operation according to a level of the supply voltage or load power.
 13. The controller according to claim 12, wherein the controller is configured to control a flyback converter that performs DC-to-DC power conversion.
 14. The controller according to claim 12, wherein the controller is configured to control the power converter so that output voltage is formed from a current induced in a secondary winding of the transformer and wherein the output voltage is regulated in a feedback loop.
 15. The controller according to claim 12, wherein the controller comprises a high side driver controller that is configured to control the high side switch wherein a signal informs the high side driver controller that the high side driver controller has permission to close the high side switch.
 16. The controller according to claim 15, wherein the controller comprises a low side driver controller that is configured to control the low side switch and wherein the low side driver controller and the high side driver controller have different ground reference voltages and wherein the signal that informs the high side driver controller that the high side driver controller has permission to close the high side switch is communicated to the high side driver controller by the low side driver via a differential signal.
 17. A switching power supply comprising: the controller according to claim 12; and a power converter comprising the transformer, the low side switch, the high side switch and the snubber capacitor.
 18. A controller configured to control a power converter, the power converter having a transformer, a low side switch for drawing current from a supply voltage through a primary winding of the transformer and a high side switch for coupling the primary winding of the transformer to a snubber capacitor, wherein a first voltage is generated at a node between the low side switch and the high side switch, and wherein the controller is configured to generate a drive signal that controls the opening and closing of the high side switch to form a regulated output voltage, wherein the controller is configured to selectively operate in a mode in which the high side switch is closed during each of a series of switching cycles based on a comparison of the supply voltage to the first voltage and in a mode in which the high side switch closed during each of a series of switching cycles in response to a pulse signal, and wherein the controller selects between the modes of operation according to a level of the supply voltage or load power.
 19. The controller according to claim 18, wherein the controller is configured to control a flyback converter that performs DC-to-DC power conversion.
 20. The controller according to claim 18, wherein the controller is configured to control the power converter so that output voltage is formed from a current induced in a secondary winding of the transformer and wherein the output voltage is regulated in a feedback loop.
 21. The controller according to claim 18, wherein the controller comprises a low side driver controller that is configured to control the low side switch and a high side driver controller that is configured to control the high side switch, wherein the low side driver controller and the high side driver controller have different ground reference voltages and wherein the signal that informs the high side driver controller that the high side driver controller has permission to close the high side switch is communicated to the high side driver controller by the low side driver via a differential signal.
 22. The controller according to claim 18, wherein a duration of the pulse signal determines a duration of time during which the high side switch is closed.
 23. The controller according to claim 22, wherein a duration of the pulse signal determines a duration of time during which the high side switch is closed.
 24. The controller according to claim 18, wherein the duration of the pulse signal is dependent upon the level of the supply voltage.
 25. The controller according to claim 24, wherein the duration of the pulse signal is adjustable by adjusting a resistor value.
 26. A switching power supply comprising: the controller according to claim 18; and a power converter comprising the transformer, the low side switch, the high side switch and the snubber capacitor. 